Descriptions

We report new major element, trace element, isotope ratio, and geochronological data on the Galapagos
Archipelago. Magmas erupted from the large western volcanos are generally moderately fractionated tholeiites of
uniform composition; those erupted on other islands are compositionally diverse, ranging from tholeiites to picritic
basanitoids. While these volcanoes do not form a strictly linear age progressive chain, the ages of the oldest dated flows
on any given volcano do form a reasonable progression from youngest in the west to oldest in the east, consistent with
motion of the Nazca plate with respect to the fixed hotspot reference frame. Isotope ratios in the Galapagos display a
considerable range, from values typical of mid-ocean ridge basalt on Genovesa (⁸⁷Sr/⁸⁶Sr: 0.70259, εNd: +9.4,
²⁰⁶Pb/²⁰⁴Pb: 18.44), to typical oceanic island values on Floreana (⁸⁷Sr/⁸⁶Sr: 0.70366, εNd: +5.2, ²⁰⁶Pb/²⁰⁴Pb: 20.0).
La/SmN ranges from 0.45 to 6.7; other incompatible element abundances and ratios show comparable ranges. Isotope
and incompatible element ratios define a horseshoe pattern with the most depleted signatures in the center of the
Galapagos Archipelago and the more enriched signatures on the eastern, northern, and southern periphery. These
isotope and incompatible element patterns appear to reflect thermal entrainment of asthenosphere by the Galapagos
plume as it experiences velocity shear in the uppermost asthenosphere. Both north-south heterogeneity within the
plume itself and regional variations in degree and depth of melting also affect magma compositions. Rare earth
systematics indicate that melting beneath the Galapagos begins in the garnet peridotite stability field, except beneath the
southern islands, where melting may occur entirely in the spinel peridotite stability field. The greatest degree of
melting occurs beneath the central western volcanos and decreases both to the east and to the north and south. Si₈.₀,
Fe₈.₀, and Na₈.₀, values are generally consistent with these inferences. This suggests that interaction between the plume
and surrounding asthenosphere results in significant cooling of the plume. Superimposed on this thermal pattern
produced by plume-asthenosphere interaction is a tendency for melting to be less extensive and to occur at shallower
depths to the south, presumably reflecting a decrease in ambient asthenospheric temperatures away from the Galapagos
Spreading Center.